I was fairly indifferent to the RAC's until, in a random game, I was assigned a Blade. Once I got the feel of that mech, it was suddenly very clear what the role of the RAC was... a mid-range brawler, and area denial weapon. But it has to be used on the correct platform. Designs, like the Blade, or the Legionnaire emphasize the use of RAC's, when used as primary weapons... get in range, and brutalize your opponent in the most unseemly way possible. Sadly, most of my group now recognizes my Blades, and does everything possible to keep me from getting behind them.......LOL.......
On designs like the "Assault Pike".... he Pike Support Vehicle with the 3 RAC/2's each, mounted on turrets, you use them like kind of like light SRM carriers with a range of 18....... use something like an Alacorn to open up the armor and then fill it with crit seeking shells, or use the volume of fire to sand armor, similar to SRM missiles.
As secondary weapons, they give you options. You can use them to either sand armor, provide crit seeking, or just add damage, as needed. At range, you can use them as normal AC's firing one or 2 shots, and lulling opponents into thinking they are just standard or ultra AC's.... and then surprise them, or just go in, burning ammo, and seeking to crush your enemy via volume of fire. Just remember, at longer ranges, using them in single shot mode still allows you to maintain an offensive posture, while still conserving ammo for those heavy bursts, and avoiding the risk of jamming a weapon while you are on the offense.
Nahuris